1. Field of the Invention
This invention relates to an air-fuel ratio control system for internal combustion engines, and more particularly to an air-fuel ratio control system of this kind, which is provided with an exhaust gas component concentration sensor in the exhaust system of the engine, and controls the air-fuel ratio of an air-fuel mixture supplied to the engine, based on results of comparison between an output value from the exhaust gas component concentration sensor and a predetermined reference value.
2. Prior Art
To reduce the amount of noxious components in exhaust gases emitted from internal combustion engines, an air-fuel ratio feedback control system is widely known, which is provided with an exhaust gas component concentration sensor arranged in the exhaust system of the engine, and controls the air-fuel ratio of an air-fuel mixture in a feedback manner responsive to an output value from the exhaust gas component concentration sensor. However, the known air-fuel ratio feedback control system has a drawback that the air-fuel ratio control cannot be carried out in a desired manner when the exhaust gas component concentration sensor undergoes a failure, such as disconnection or short-circuit of an electric system of the exhaust gas component concentration sensor.
To overcome the above drawback, a checking system for the air-fuel ratio control system has been proposed, for example, by U.S. Pat. No. 4,149,408, which is provided with an abnormality-detecting circuit for detecting abnormality of the exhaust gas component concentration sensor. When an abnormality of the exhaust gas component concentration sensor is detected by the abnormality-detecting circuit, the proposed checking system operates to inhibit the feedback control of the air-fuel ratio and light up an alarm lamp to inform the driver of the abnormality.
According to the proposed checking system, since the feedback control of the air-fuel ratio is thus inhibited when an abnormality of the exhaust gas component concentration sensor is detected by the abnormality-detecting circuit, degradation of the drivability of the engine ascribable to erroneous correction of the fuel amount to be supplied to the engine can be prevented.
The proposed checking system detects not only abnormality of the exhaust gas component concentration sensor caused by disconnection and short-circuit of the electrical system of the sensor but also abnormality of the sensor due to aging thereof, such as degradation of the responsiveness of the sensor. When such abnormality due to aging is detected, however, it is sometimes better to continue the feedback control of the air-fuel ratio than to inhibit the same, depending on the deterioration degree of the exhaust gas component concentration sensor, to maintain required drivability and exhaust emission characteristics of the engine.
However, the proposed checking system inhibits execution of the feedback control even when the deterioration degree of the sensor is so small that exhaust gases can still be purified to a sufficient degree by a catalytic converter arranged in the exhaust pipe at a location downstream of the sensor. As a result, the inhibition of the feedback control under such circumstances even results in further degraded exhaust emission characteristics of the engine.